/**

* 文本框根据输入内容自适应高度

* @param                {HTMLElement}        输入框元素

* @param                {Number}                设置光标与输入框保持的距离(默认0)

* @param                {Number}                设置最大高度(可选)

*/

var autoTextarea = function (elem, extra, maxHeight) {

    extra = extra || 0;

    var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,

        isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),

        addEvent = function (type, callback) {

            elem.addEventListener ?

                elem.addEventListener(type, callback, false) :

                elem.attachEvent('on' + type, callback);

        },

        getStyle = elem.currentStyle ? function (name) {

            var val = elem.currentStyle[name];



            if (name === 'height' && val.search(/px/i) !== 1) {

                var rect = elem.getBoundingClientRect();

                return rect.bottom - rect.top -

                    parseFloat(getStyle('paddingTop')) -

                    parseFloat(getStyle('paddingBottom')) + 'px';

            };



            return val;

        } : function (name) {

            return getComputedStyle(elem, null)[name];

        },

        minHeight = parseFloat(getStyle('height'));



    elem.style.resize = 'none';



    var change = function () {

        var scrollTop, height,

            padding = 0,

            style = elem.style;



        if (elem._length === elem.value.length) return;

        elem._length = elem.value.length;



        if (!isFirefox && !isOpera) {

            padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));

        };

        scrollTop = document.body.scrollTop || document.documentElement.scrollTop;



        elem.style.height = minHeight + 'px';

        if (elem.scrollHeight > minHeight) {

            if (maxHeight && elem.scrollHeight > maxHeight) {

                height = maxHeight - padding;

                style.overflowY = 'auto';

            } else {

                height = elem.scrollHeight - padding;

                style.overflowY = 'hidden';

            };

            style.height = height + extra + 'px';

            scrollTop += parseInt(style.height) - elem.currHeight;

            document.body.scrollTop = scrollTop;

            document.documentElement.scrollTop = scrollTop;

            elem.currHeight = parseInt(style.height);

        };

    };



    addEvent('propertychange', change);

    addEvent('input', change);

    addEvent('focus', change);

    change();

};
